Phone I think you are missing the point here.
The school obviously have good reason to put a blanket ban in place over misuse of mobile phones in school. There are so many scenarios of where children and parents could argue "but they were only doing xyz.." which is why it is probably a blanket ban.
They are school rules and every pupil has to abide by them or, like anything in real life, they have to accept the consequences.
If they said "oh ok you were only doing that, here's your phone," then it would completely undermine their rules and authority as a whole.
Kids need to learn because when they enter into employment, Mum and dad won't be there to "kick off" at the workplace about "how unfair" the companies policy on certain things are.
Part of being at school is preparing them for future employment and real life. If you go and get the phone and give it to your dc then all that is teaching them is that it's ok to break school rules and they won't respect them in future whereas if the confiscation is followed through it's more than likely they will think twice next time.
Support the school. It creates better adults.